The splenic component of mononuclear phagocytic cell function was investigated in rabbits using as a marker the clearance of N-ethylmaleimide-treated, technetium-labelled autologous erythrocytes. In 66 clearances performed in 23 normal rabbits, the clearance T1/2 was found to be between 5 and 16 min, the value for each rabbit remaining relatively constant. Clearance was proportional to incubation time or the dose of N-ethylmaleimide used. Clearance was delayed following injection of 20 to 65 mg of preformed complexes of BSA-anti-BSA made in 10-fold antigen excess. The degree and duration of this blockade was related to the dose of the complexes up to a critical value of 20 mg (antibody content after which no further decrease in clearance occurred although the duration of the blockade was longer at higher doses. Ultracentrifugation studies showed that these complexes were about 14S in size and sequential studies revealed that they were slowly cleared from the circulation. The experiments indicate that studies of splenic function may be of value in assessing immune complex disease, since circulating immune complexes of these characteristics are poorly detected by current methodology.